(1) Field of the Invention
This invention relates to an optical transmission apparatus and, more particularly, to an optical transmission apparatus for amplifying and transmitting an optical signal.
(2) Description of the Related Art
Large capacity and high speed are required in optical communication, so expectations for wavelength division multiplexing (WDM) transmission are growing. WDM multiplexes a plurality of optical signals with different wavelengths into a single optical fiber.
Erbium-doped fiber amplifiers (EDFAs) are widely used for optical amplification in WDM systems. EDFAs are optical amplifiers using an erbium-doped fiber (EDF) as an amplification medium. With EDFAs, an EDF through which an optical signal travels is irradiated with excitation light and the level of the optical signal is amplified by stimulated emission caused by the irradiation. A wide-band gain spectrum is obtained by each EDFA, so a plurality of optical signals with wavelengths included in this band can be amplified in block. EDFAs therefore are main devices in WDM repeaters.
Moreover, in recent years optical fiber amplifiers called Raman amplifiers have been put to practical use. With Raman amplifiers, strong excitation light is inputted to the entire optical fiber transmission line to perform optical amplification. To be concrete, the Raman stimulated scattering effect of an optical fiber is utilized and 1.5 μm band optical signals are amplified by the use of 1.4 μm band excitation light. By using Raman amplifiers in repeaters, repeating station spacing can be widened and long-distance large-capacity optical transmission can be realized.
By the way, with optical amplifiers, such as EDFAs or Raman amplifiers, in which optical signals are amplified on the basis of stimulated emission or stimulated scattering, the phenomenon of spontaneous emission or spontaneous scattering will occur regardless of whether there is an input optical signal. Light which leaks out from optical amplifiers as a result of these phenomena becomes noise and is called amplified spontaneous emission (ASE) for amplification by EDFs or amplified spontaneous scattering (ASS) for Raman amplification.
To control optical amplification, the optical level of a main signal itself must be detected with great precision regardless of the amount of such optical noise and optical amplification must be performed on the basis of the result of the detection. Detection of the optical level of a signal including a noise component, that is to say, detection of the sum of the level of a main signal and the level of the noise component will cause a transmission error.
It is assumed that the level of a main signal itself is low and that the level of a noise component is high. If the level of an optical signal to be transmitted is considered to satisfy a desired value and this optical signal is transmitted to a downstream station, then problems will arise. For example, the level of the main signal the downstream station can receive cannot be ensured. As a result, a transmission error occurs or repeating station spacing cannot be widened.
Conventionally, to control optical amplification, the technique of comparing the intensity level of ASE noise light which is emitted from a rare-earth doped fiber, such as an EDF, with an intensity level at which predetermined amplification factor dependence is obtained and of controlling the amount of excitation light so that the two intensity levels will match has been proposed (see, for example, Japanese Unexamined Patent Publication No. 7-193542, paragraph nos. [0032]-[0033] and FIG. 11).
With the above conventional technique, however, only EDFs are considered as optical amplification media. Therefore, it does not apply to Raman amplifiers. Conventionally, on the other hand, to perform an optical noise correction, the amount of optical noise is measured in advance in a test environment and a correction is performed in an operating environment on the basis of the level of the optical noise measured.
That is to say, first, a predetermined excitation light is inputted to an optical amplification medium in a test environment and the level of optical noise produced is measured. In an operating environment, the level of a main signal itself is detected by subtracting the value of the optical noise level measured in advance from the level of an optical signal amplified by an optical amplifier (from the level of an optical signal obtained by amplifying an input optical signal by inputting the predetermined excitation light used in the test environment to the optical amplification medium).
With such conventional optical noise correction, however, the difference between a test environment in which data regarding optical noise is acquired and a customer's operating environment will cause an error in the amount of noise. Accordingly, optical noise cannot be corrected accurately and the level of a main signal itself cannot be detected accurately.
An optical fiber transmission line itself is used as an amplification medium especially in Raman amplifiers. Therefore, an error will occur between ASS measured in a test environment and ASS which occurs in an operating environment because of a difference in the WDL of a fiber (the wavelength characteristic of a fiber loss), a lamp loss (a loss which will occur around a portion of a transmission line in a Raman amplifier where excitation light is inputted), or the like. In addition to an error which will occur in a sample optical amplifier at the time of data acquisition, errors may be caused by, for example, variations (part variations) among optical amplifiers manufactured and shipped.
It is assumed that a correction amount is larger than the actual amount of noise (that a value greater than the actual level of optical noise is subtracted from the level of an optical signal amplified). Even if the optical signal sent from an upstream station is at a level a downstream station can receive, the level of a main signal will be estimated at a small value at the downstream station. This may lead to the judgment that the optical signal cannot be transmitted from the downstream station. (In an extreme case, the judgment that there is no main signal (that the circumstances are in an alarm state) is made and output from an optical amplifier is stopped.) By contrast, it is assumed that a correction amount is smaller than the actual amount of noise (that a value smaller than the actual level of optical noise is subtracted from the level of an optical signal amplified). Even if the optical signal sent from an upstream station is at a level a downstream station cannot receive, the level of a main signal will be estimated at a great value at the downstream station. This may lead to the judgment that the optical signal can be transmitted from the downstream station. (In an extreme case, though there is no transmitted main signal due to a failure, such as disconnection of a fiber, the optical noise may be considered as a main signal and be transmitted from the downstream station.) These two defects will appear significantly in the case of the number of wavelengths being small. As a result, the limitation of the minimum operating wavelength number is imposed.